Radioactivity was discovered in 1896, quickly spurring research in countries
burgeoning during the industrial revolution. Scientists, including Marie
Curie in France and Ernest Rutherford in England, restlessly searched
for radioactive minerals and conducted lab experiments on their properties.

Before the 20th century, the most commonly accepted model of the atom
placed electrons inside a positively charged fluid. The negative electrons
floated in the fluid and occasionally escaped as radioactivity.

Around the turn of the century, that idea had to be changed. Ernest Rutherford
and his students H. Geiger and E. Marsden made a fundamental discovery
revealing the structure of the nucleus. Their experiments indicated that
there was a tightly packed and positively charged core in the atom and
the electrons orbited around it.

In 1913, Neils Bohr came up with a model that fit much of the experimental
observations. Bohr theorized that electrons orbited the nucleus in set
orbits, instantaneously jumping from orbit to orbit when the atom lost
or gained energy.

The secrets of the nucleus were harder to penetrate. Rutherford suggested
that the nucleus had a positively charged proton and a neutral particle
he called a neutron. In 1932, Chadwick in England found the neutron. That
same year, Kurchatov shifted his field of research to nuclear physics.

The potential for a new energy source was quickly realized. The next
year, physicists worked on producing a chain reaction of uranium. By the
end of the year, Szilard and Einstein sent a letter to President Roosevelt
warning him of the potential for a new weapon.

Before the first atomic bomb was built using fissionable material, both
the Americans and Soviets were working on a more powerful fusion bomb.

Fusion

Fusion occurs when nuclei fuse together -- a reaction that includes emission
of energy. However, this reaction can only occur under extremely high
temperatures, reaching the temperature of the interior of stars. Nuclei
have a strong repulsive force against each other because of their positive
charge, so they need to be very energetic to overcome the repulsion.

Because fusion uses elements such as hydrogen isotopes, and it results
in non-radioactive products such as helium, fusion power holds great
promise. However, because of the incredibly high temperatures required,
a form of containment needs to be developed before a fusion reactor can
be built. Sakharov's Tokamak is one potential reactor, using powerful
magnetic fields.